# C++ named requirements: RangeAdaptorClosureObject (since C++20)

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Range adaptor closure objects are FunctionObjects that are callable via the pipe operator: if C is a range adaptor closure object and R is a `range`, these two expressions are equivalent:

    C(R)
    R | C

Two range adaptor closure objects can be chained by operator| to produce another range adaptor closure object: if C and D are range adaptor closure objects, then C | D produces a range adaptor closure object E with the following properties:

- E stores a copy of C and D, direct-non-list-initialized from std::forward<decltype((C))>(C) and std::forward<decltype((D))>(D) respectively. If such initialization is invalid, C | D is also invalid.
- Let c and d be the stored copies (with the same constness and value category as E), and R be a `range` object, the following expressions are equivalent:

    d(c(R))
    R | c | d
    E(R)
    R | E // R | (C | D)

Notes: operator() is unsupported for volatile-qualified or const-volatile-qualified version of range adaptor object closure types.

Objects whose type is the same as one of the following objects (ignoring cv-qualification) are range adaptor closure objects:

- unary range adaptor objects,

objects of user-defined types that meet the requirements of implementing a range adaptor closure object, | (since C++23)

- the results of binding trailing arguments by range adaptor objects, and
- the results of chaining two range adaptor closure objects by operator|.

### See also

ranges::range_adaptor_closure (C++23) | helper base class template for defining a range adaptor closure object (class template)

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